Synchronous Machines

Enclosed motors with air-to-water coolers cost much less than TEF( motors above 373 kW (5()0 hp) in large svnchronoiis-rnotor ratings, thev cost even less than weather-protected, tvpe II, Large enclosed synchronous machines with coolers for mounting in the motor foundation are frequently supplied at lower cost than motors with integral-mounted coolers. 

Rotating electrical machines Specific requirements for turbine type synchronous machines BS EN 60034-3/1996 ... 

Consider a process plant having a connected load of 15 000 kW and a running load of 12 500 h.p. at almost 0.65 p.f. lagging. Let a few large induction motors aggregating 2000 h.p. be replaced by as many oversized synchronous machines, with the purpose of improving the system p.f. in addition to performing the motor s duties. 

Rotating electrical machines Specific requirements for turbine type synchronous machines Specification for voltage regulation and parallel operation of a.c. synchronous generators — BS F.N 60034-3/1996 BS 4999-140/1987... 

ANSI/IEEE 115-1/1995 Test procedures for synchronous machines, acceptance and performance testing ... 

This is a very good motor for direct connection to certain loads, particularly where constant speed is required. NEMA defines it as a synchronous machine which transforms electrical power from an alternating-current system into mechanical power. It usually has direct-current field excitation by a separately driven direct-current generator or one directly connected to the motor. This motor remains synchronous with the supply frequency and is not affected by the load. Proper application requires consideration of the following ... 

Synchronous Motor. A synchronous motor is a synchronous machine which transforms electrical power into mechanical power. 

Since synchronous machines represent the most common type of generator used in power installations, the remainder of this section is principally concerned with this type. 

The power factor of an installation can be improved by the use of either A.C. synchronous machines or of static capacitor banks. An A.C. synchronous machine will either draw current from the supply or contribute current to the supply, depending on whether the machine is operating ... 

When an A.C. synchronous machine operating as a motor in parallel with other loads and an external supply system is over-excited the machine will contribute reactive kvar to the supply. The net effect of this will be to reduce the amount of reactive current drawn from the supply, and this will improve the overall system power factor. 

However, the power factor of the current drawn from the supply will only improve if the power factor of the generated current is less than that of the parallel loads, since the active power drawn from the supply will also be reduced by the amount of actual power generated. In each case increasing the excitation so that the power factor of the current drawn from the supply can improve to unity or become leading can increase the reactive current produced by the synchronous machine within the capabilities of the machine. In this case, the installation becomes a net exporter of lagging reactive current to the supply. Figure 16.8 illustrates these two cases. 

The second method of improving the power factor of an installation is to provide static capacitor banks. These can be installed as a single block at the point of supply busbar, as a set of switchable banks or as individual units connected to specific loads. For an installation where no synchronous machines are installed for other purposes (i.e. as prime movers or generators) then static capacitor banks are almost invariably the most cost-effective way of improving the power factor. 

Figure 16.8 Synchronous machine operation, (a) Synchronous machine acting as a motor (over-excited) (b) synchronous machine acting as a generator (over-excited)...

Fig. 5.6 Schematic illustration of synchronous machines of a round or cylindrical rotor and b salient rotor structures...

Variable reluctance synchronous AC machines represent other types of synchronous machines which do not require any excitation system on the rotor [3]. 

The rotating held in the air gap of a synchronous machine is generally considered to be free of space harmonics, when the basic operation of the machine is being considered. In an actual machine there are space harmonics present in the air gap, more in salient pole machines than a cylindrical rotor machine, see for example References 4 and 6. It is acceptable to ignore the effects of space harmonics when considering armature reaction and the associated reactances. Therefore the flux wave produced by the rotating field winding can be assumed to be distributed sinusoidally in space around the poles of the rotor and across the air gap. 

Edward Wilson Kimbark, Power system stability synchronous machines. Dover Publications, Inc. (1968). Library of Congress Card No. 68-12937. 

A synchronous generator (and a synchronous motor) can be represented by many inductances and reactances to account for transformer-type induction, rotational induction, mutual coupling between windings, leakage and self-induction, magnetising and excitation induction and the effects of the pole-face damper windings. Extremely complex equivalent circuits have been developed for synchronous machines, see References 1 and 2 as examples. 

During a fault condition, the load side of the power system can contribute currents to the fault. The origin of such contribution is motors, which can be either induction or synchronous machines. 

Induction motors can be represented by the 2-axis theory, by using the derivations for synchronous machines but deleting the field winding. In this case some of the reactances become zero, and the field resistance is infinity. Hence, the derived reactances X ... 

In an interconnected power system there will be two or more synchronous machines (or groups of machines). These machines will be coupled through their own internal reactances and through... 

A. W. Rankin, The direct and quadrature axis equivalent circuits to the synchronous machine. AIEE Transactions, Vol. 64, December 1945, pages 861 to 868. 

T. J. Hammons and D. J. Winning, Comparison of synchronous machine models in the study of the transient behaviour of electrical power systems. Proc lEE, Paper No. 6469P, Vol. 118, No. 10, Oct 1971. 

These equations can be used to determine the initial conditions of the synchronous machine in a computer program. 

At this stage operational impedances and time constants have been derived for synchronous machines, and for induction machines, if appropriate substitutions are made as shown in Reference 23. 

The following discussion applies to a synchronous machine that has one field and two damper windings. 

The absence of the field winding can be used to convert the mathematical model of the synchronous machine into one for an induction machine. In addition the mutual inductance in the < -axis is made equal to mumal inductance in the d-axis, i.e. the machine becomes symmetrical in both axes. The matrix equations (20.6) to (20.16) are modified as shown below. In these equations the mutual inductances Mj and become M, Lim and Lihq become L/j., Rjut and R/aj become Rk. All the derived reactances and time constants for an induction machine are equivalent to those appUcable to the g-axis of the synchronous machine. 

Application of a three-phase short circuit to the terminals of an unloaded induction motor is not a practical factory test, especially for a large high-voltage motor, because the motor can only be excited at its stator windings from the power supply. A three-phase short circuit at or near the stator terminals can occur in practice e.g. damaged supply cable, damage in the cable terminal box. The parameters of the stator and rotor windings can be obtained from other factory tests. However, the derived reactance can be defined in the same manner as those for the synchronous machine, but with... 

Equation (20.23) can be rewritten with the rotationally induced emfs correctly represented by the rotor speed cOr instead of co as in the case of the synchronous machine -... 

R. H. Park, Two-reaction theory of synchronous machines. Generalised method of analysis. Part 1. Transactions ofAIEE, Vol. 48, 1929, page 716. Parti. Transactions of AIEE, Vol. 52, 1933, page 352. 

C. Concordia, Synchronous machines. John Wiley Sons Inc. New York (1951). 

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